When you need to make a thread repair, and someone has “borrowed” your tap drill size chart, here is a little trick. Subtracting the pitch from the nominal thread O.D. will get you very close to the tap drill size for a 75% thread. Here are some examples:
Metric Thread
M6 x 1.0 tap drill is 6mm – 1mm = 5mm / 0.1969″
M8 x 1.25 tap drill is 8mm – 1.25mm = 6.75mm / 0.2657″
Imperial Thread
A 1/4-20 tap drill is 0.250″ – 1/20 or 0.050″ = 0.200″. Use a #7 drill bit 0.201″.
A 5/16-18 tap drill is 0.3125″ – (1/18) = 0.2569″. Use a Letter ‘F’ drill bit (0.257).
A 3/8-16 tap drill is 0.375″ – (1/16) = 0.3125″. Use a 5/16″ bit.
Tom Nichols
Automotive Machine & Supply, Inc
Joshua, TX
